TKR via subvastus (Southern) approach: consultant-style viva prep

1. “What is the subvastus approach?”

2. “Why choose it over a medial parapatellar approach?”

• Preserves extensor mechanism integrity, potentially allowing earlier quadriceps recovery and straight-leg raise.
• Preserves the intramuscular articular branch of the descending genicular artery, supporting patellar blood supply.
• May improve early postoperative pain, active extension, and rehabilitation.
• Avoids splitting the quadriceps tendon and VMO.

3. “What are its main disadvantages?”

• Limited exposure, particularly of the lateral compartment and lateral femoral condyle.
• Patellar eversion can be difficult. Lateral subluxation rather than forceful eversion is often safer.
• Technically harder in muscular, obese, stiff, or severely deformed knees.
• Can require conversion or proximal extension for safe visualization.
• Excess traction can injure the VMO, medial retinaculum, patellar tendon, or compromise the skin flap.

4. “What are relative contraindications?”

• Previous major knee arthroplasty or substantial prior scar/distorted anatomy.
• Obesity or a large thigh girth. Campbell’s describes thigh girth of about >55 cm as a practical problem because patellar eversion may not be possible.
• Markedly stiff knee or severe flexion contracture.
• Complex primary TKR needing broad exposure, such as severe deformity, major bone loss, difficult valgus balancing, or anticipated constrained implant.
• Revision TKR.

5. “Describe the incision and superficial dissection.”

1. Supine position, tourniquet according to the surgeon’s preference.
2. Midline anterior skin incision, from roughly 8 cm proximal to the patella to just medial and about 2 cm distal to the tibial tubercle.
3. Develop the medial flap carefully. Be aware of the variable infrapatellar branch of the saphenous nerve, which is commonly divided with an anterior incision but should not be aggressively sought or damaged further.
4. Incise superficial fascia just medial to the patella.
5. Identify the inferior border of VMO and elevate it from the periosteum/intermuscular septum, preserving muscle continuity.

6. “How far proximally do you elevate the VMO?”

7. “What arthrotomy do you make?”

• Start through the VMO tendinous insertion into the medial patellar retinaculum.
• Continue distally along the medial border of the patella.
• Then proceed medially beside the patellar tendon as necessary for exposure.

8. “How do you mobilize the patella?”

• Elevate the VMO anteriorly.
• Release only enough medial patellotibial/retinacular tissue to allow safe mobilization.
• A limited medial patellar tendon release may be used in the classic technique.
• With the knee extended, evert laterally only if it is easy and safe.
• If tight, do not force eversion. Lateral subluxation with the knee flexed is often adequate.

9. “What are the key structures at risk?”

10. “Why is patellar vascularity relevant?”

11. “When might you need a lateral release?”

• femoral component internal rotation
• tibial component internal rotation
• excessive patellar thickness after resurfacing
• malpositioned patellar button
• tight lateral structures or unbalanced flexion/extension gaps

12. “How do you assess patellar tracking?”

• Bring the knee through range of motion.
• Observe central patellar tracking in the trochlea.
• Assess tilt, subluxation, and the need for manual medial pressure.
• Use the surgeon’s preferred “no-thumb” or passive tracking assessment, without forcing the patella.

13. “What do you do if exposure is inadequate?”

“I would not persist with poor visualization. I would release adhesions judiciously, use lateral patellar subluxation rather than forceful eversion, and extend the arthrotomy proximally between rectus femoris and VMO if needed. In a difficult stiff or revision situation, I would use an extensile exposure appropriate to the case, such as a quadriceps snip or tibial tubercle osteotomy under consultant direction.”

14. “Compare subvastus, midvastus, and medial parapatellar.”

15. “What are key closure points?”

• Meticulous capsular and medial retinacular repair.
• Restore the VMO/retinacular layer without overtightening.
• Confirm tracking and full extension before final closure.
• Hemostasis, especially around the VMO bed and medial gutter.
• Layered closure with careful skin handling.

Questions you may be asked while assisting

Practical assistant checklist for tomorrow

• Know the patient’s diagnosis, deformity, ROM, flexion contracture, prior scars, BMI, patellar height, and whether resurfacing is planned.
• Identify the VMO, medial patellar retinaculum, patellar tendon, tibial tubercle, medial femoral epicondyle, and adductor tubercle before incision.
• During exposure: anticipate medial retractors, Z-retractor/Hohmann positioning, suction, cautery, and controlled lateral patellar subluxation.
• Never retract aggressively against the patellar tendon or VMO.
• During trialing, be ready to state: “I would check component rotation first, then patellar thickness and tracking, before considering lateral release.”
• If unsure, prioritize the consultant’s specific implant system, workflow, and exposure preference over generic technique descriptions.

1. What is the defining principle of the subvastus approach?
2. Which part of quadriceps is mobilized, and what is specifically not divided?
3. Why may subvastus preserve patellar vascularity better than medial parapatellar arthrotomy?
4. Which arterial branch is classically preserved?
5. Describe the arthrotomy shape and course.
6. What is the key technical difficulty of this approach?
7. In a tight knee, what should you avoid doing to the patella?
8. If patellar eversion is difficult, what is the safer alternative?
9. Give three relative contraindications or situations where you would be cautious.
10. Why is a large thigh girth problematic in this approach?
11. What anatomical plane is developed to mobilize the VMO?
12. What landmark is relevant for the proximal extent of VMO elevation?
13. Roughly how far proximal is the VMO elevated in the classic description?
14. What extensor-mechanism structure is especially at risk during difficult exposure?
15. What should you check before deciding on a lateral retinacular release?
16. Name four causes of poor patellar tracking after TKR other than a tight lateral retinaculum.
17. Why can lateral release be problematic in a resurfaced patella?
18. If visualization is inadequate, what is the correct principle?
19. How can the exposure be extended proximally if required?
20. Compare the main difference between subvastus and midvastus approaches.

• Obesity / large thigh girth
• Previous surgery, scars, or distorted tissue planes
• Severe deformity, especially fixed valgus or a stiff contracted knee, when wide exposure may be needed

• Femoral component rotation, especially unintended internal rotation
• Tibial component rotation
• Component size and position
• Patellar resection symmetry, residual patellar thickness, and button position
• Flexion/extension balance and lateral soft-tissue tightness

• Subvastus: VMO is elevated off the medial intermuscular septum, with no muscle split.
• Midvastus: muscle fibers of the VMO are split to gain exposure.

• Femoral component internal rotation
• Tibial component internal rotation
• Incorrect femoral component size/position can also contribute.

• Patellar tendon injury or avulsion, sometimes with a tibial-tubercle avulsion fragment
• Injury to the quadriceps tendon/VMO or broader extensor mechanism
• Potential additional compromise of patellar vascularity, particularly if combined with extensive releases

• Tight lateral retinaculum
• Lateral peripatellar adhesions/scarring
• Medial retinacular or MPFL insufficiency, allowing lateral displacement

Benefits

1. Quadriceps and extensor-mechanism preservation - less disruption of VMO/quadriceps tendon, supporting early activation and rehabilitation.
2. Better preservation of patellar vascularity - particularly the articular branch of the descending genicular artery.

Limitations

1. Restricted exposure, particularly difficult patellar mobilization/eversion and visualization of the lateral side of the knee.
2. Technically demanding in stiff, obese, muscular, scarred, or severely deformed knees.

Final score: 170/200 = 85%

Consultant-ready one-liner

“The subvastus approach is a quadriceps-sparing exposure that preserves VMO continuity and patellar blood supply, potentially aiding early recovery, but it has more limited exposure and can make patellar mobilization difficult in stiff, obese, or complex knees.”

MCQ 1/10: Coronal alignment in mechanically aligned TKR

• Femoral and tibial components are positioned perpendicular to their respective mechanical axes.
• The target overall hip-knee-ankle alignment is approximately 180°, or neutral.

MCQ 2/10: Femoral component rotation